This application claims the priority of Korean Patent Application No. 2003-14613, filed on Mar. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser diode. More particularly, the present invention relates to a submount flip-chip bonded to a semiconductor laser diode chip having two stepped electrodes that are formed on the same side, a method of manufacturing the same, and a semiconductor laser diode assembly using the submount.
2. Description of the Related Art
As high-density information recording is increasingly being required, the demand for a visible light semiconductor laser diode is increasing. Therefore, semiconductor laser diodes made of various compounds capable of emitting a visible light laser are being developed. In particular, much attention has been paid to a group III-V nitride semiconductor laser diode because of its optical transition being of a direct transition type that induces high frequency laser emission and since it emits a blue light laser.
FIG. 1 shows a sectional view of a conventional GaN-based, group III-V nitride semiconductor laser diode chip having n-type and p-type electrodes, which are formed on the same side, and a ridge waveguide.
As shown in FIG. 1, a conventional GaN-based, group III-V nitride semiconductor laser diode chip has n-type and p-type electrodes, which are formed on the same side, and a ridge waveguide formed in the p-type electrode region.
In detail, an n-GaN layer 12 is formed on a sapphire substrate 10. The n-GaN layer 12 is divided into first and second regions R1 and R2. An n-type electrode 14 is formed on the second region R2 of the n-GaN layer 12. An n-AlGaN/GaN layer 16, an n-GaN layer 18, and an InGaN layer 20 as an active layer are subsequently formed on the first region R1 of the n-GaN layer 12 in sequence from smaller to larger refractive index. A p-GaN layer 22, a p-AlGaN/GaN layer 24, and a p-GaN layer 26 are formed on the InGaN layer 20 in sequence from larger to smaller refractive index. The upper central portion of the p-AlGaN/GaN layer 24 is protruded in the form of a ridge or rib and the p-GaN layer 26 is formed on top of the ridge. The p-AlGaN/GaN layer 24 is covered with a protective layer 28 having a channel 27 which communicates with the p-GaN layer 26. A p-type electrode 30 is formed on the protective layer 28 and the exposed middle surface of the p-GaN layer 26, and becomes in contact with both ends of the p-GaN layer 26 through the channel 27. In this structure, the p-type electrode 30 and the n-type electrode 14 are separated by a step height, h1.
Generally, a temperature has an effect on a critical current and laser mode stability for laser emission of semiconductor laser diodes. As a temperature increases, both of the characteristics are lowered. Therefore, there is a need to remove heat generated in the active layer during laser emission to thereby prevent overheating of laser diodes. In the case of using the structure of the aforementioned conventional GaN-based, group III-V semiconductor laser diode, most heat is discharged only through a ridge because of very low thermal conductivity of a substrate (for a sapphire substrate, about 0.5 W/cmK). However, because heat discharge through a ridge occurs limitedly, it is difficult to carry out efficient heat discharge. Therefore, lowering of characteristics of semiconductor devices that is caused by overheating of laser diodes is not efficiently prevented.
In this regard, a flip-chip bonding technology shown in FIG. 2 can be applied to the structure shown in FIG. 1 to discharge heat generated in an active layer.
Referring to FIG. 2, a reference numeral 50 indicates a semiconductor laser diode chip, which has an inverted structure of the conventional GaN-based, group III-V semiconductor laser diode shown in FIG. 1. A reference numeral 40 indicates a submount, a reference numeral 41 a substrate, and a reference numerals 42a and 42b first and second metal layers, respectively. A reference numerals 44a and 44b indicate first and second solder layers, which are respectively fused to an n-type electrode 14 and a p-type electrode 30 of the semiconductor laser diode chip 50.
By bonding a semiconductor laser diode to a separately prepared heat discharge structure shown in FIG. 2, heat discharge efficiency can be increased.
However, as shown in FIG. 2, the first and second solder layers 44a and 44b have different thicknesses in order to compensate for the step height, h1 between the n-type electrode 14 and the p-type electrode 30. That is, supposing that the thickness of the first metal layer 42a is the same as that of the second metal layer 42b, the first solder layer 44a is thicker than the second solder layer 44b by the height of h1. In this case, because the first and second solder layers 44a and 44b are not uniformly molten when respectively bonded to the two electrodes 14 and 30, there is a difference between the bonding states. FIG. 3 is a photograph showing the molten states of solder layers in the conventional submount shown in FIG. 2. As shown in FIG. 3, the first solder layer 44a and the second solder layer 44b run down while being molten non-uniformly.
The first and second solder layers 44a and 44b must have the same chemical composition ratio. Even if the chemical composition ratios of the first and second solder layers 44a and 44b slightly differ from each other, there is a large difference between their melting temperatures. As a result, the first solder layer 44a and the second solder layer 44b are not concurrently molten when bonded to the two electrodes 14 and 30, respectively, thereby causing a difference between the bonding states. In this regard, there is a need to concurrently form the first and second solder layers 44a and 44b under the same process.
As mentioned above, however, the first and second solder layers 44a and 44b differ in thickness. Therefore, in a method of manufacturing a submount, the first and second solder layers 44a and 44b cannot be concurrently formed. Rather, the two solder layers must be formed one after the other. For this reason, there exists a high likelihood for the first and second solder layers 44a and 44b to have different chemical composition ratios.
As mentioned above, if a bonding state between the first solder layer 44a and the n-type electrode 14 is different from that between the second solder layer 44b and the p-type electrode 30, heat generated upon operation of the semiconductor laser diode chip 50 is not efficiently delivered to the submount 40 thereby lowering heat discharge characteristics. As a result, heat within the active layer 20 is not sufficiently discharged. Consequently, the temperature of the semiconductor laser diode chip 50 increases and laser emission characteristics of the active layer 20 is lowered.